VIBRATIONAL ANALYSIS OF PEPTIDES, POLYPEPTIDES, AND PROTEINS - CHARACTERISTIC AMIDE BANDS OF BETA-TURNS

Normal vibration calculations were made for a type I .beta.-turn of CH3-CO-(Ala)4-NH-CH3 and a type II .beta.-turn of CH3-CO-(Ala)2-Gly-Ala-NH-CH3. The force field was the one which was refined for .beta.-sheet and .alpha.-helical structures. A calculation was also done for CH3-O-CO-Gly-(Ala)2-Gly-O-CH3, which is an appropriate model for 2 tetrapeptides for which IR data are available. The agreement between observed and calculated frequencies in this case is good, thus supporting the conclusions drawn from the above .beta.-turn calculations. The most important result of the calculations is the prediction of bands near 1690 cm-1, a region heretofore associated only with the antiparallel-chain pleated sheet structure. Thus bands observed in proteins near 1690 cm-1 should be associated with the presence of .beta.-turns as well as of .beta.-sheets. A band near 1665 cm-1 is also characteristic of type II turns.